Mechanically-produced pulps, softwood and thermomechanical pulps containing lignin, as opposed to chemically produced wood pulps, used for the production of paper, have traditionally been bleached to improve the whiteness thereof. Newsprint, which contains a relatively high lignin content is either not bleached or only mildly bleached, with the result that it is usually of a darker quality than paper produced from fully bleached pulp, and tends to darken further when exposed to light, a phenomenon known as "reversion".
Whiteness or "brightness" of paper is conventionally measured by brightness measurements based on the reflectance of light at a wavelength of 457 nm, using such instruments as an Elrepho brightness meter. There is, however, another measure of whiteness which is even more significant and that is the degree of yellowness (CIE yellow colour coordinate b*). Bleached softwood pulps usually have a brightness of about 70-90% Elrepho and a yellowness b* of 8 or more, and there are several known methods for achieving these results, using either an oxidative process using hydrogen peroxide under strongly basic conditions or reductive processes using hydrosulfite (dithionite) or combinations thereof. Attention is also directed to U.S. Pat. No. 5,080,754, issued Jan. 14, 1992, to Francis et al., which describes the use of alkali formates (Na, Mg and Ca formates) and compounds having a formyl functionality, RCHO, including formamides, formic acid esters and formylurea, to improve brightness reversion of bleached mechanical and semi-mechanical pulps and papers. This patent, however, specifically excludes formic acid and formaldehyde and there is no attempt to either increase initial ISO brightness or to reduce b* values.
There is an ongoing need for improved but inexpensive mechanical and thermo-mechanical pulps having improved brightness and decreased b* values; and with greater stability of the optical properties, i.e. decreased reversion.